Ammonia oxidation catalysts

ABSTRACT

Ammonia oxidation catalyst units comprising a pair of honeycomb-type blocks having interplaced between them a layer of a gas permeable material performing the function of radially mixing the gas flow, said blocks comprising an ammonia oxidation catalysts, and having height of less than 15 cm and the interplaced layer height of 3 to 0.5 cm.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/640,266, filed Dec. 17, 2009, the entire content and disclosure ofwhich is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an ammonia oxidation catalyst unitformed of a pair of honeycomb-type blocks comprising an ammoniaoxidation catalyst and having interplaced between them a layer of foamedmaterial.

For many years, the catalysts for ammonia oxidation have been formed ofmeshes or gauzes of platinum or its alloy with other precious metals.

Such catalysts have good activity and selectivity but suffer from thedisadvantage that the catalyst is not only very expensive but also itexhibits an appreciable loss of platinum at the high temperatures of theoxidation reaction with consequent low catalyst life, which requiresfrequent replacement.

It is therefore desirable to provide a replacement of such preciousmetal catalyst.

It is well known that oxides of metals such as manganese, iron, nickelor, especially cobalt, often used in conjunction with one or more rareearth metal oxides, exhibit activity for ammonia oxidation.

CN-A-86/108 985 describes a catalyst composition of formulaLa_(1-x)CeCoO₃ (where x is a number from 0 to 1) having perovskitestructure, endowed with good activity and selectivity when tested onsmall scale, which decreases when operating at temperatures (800°-1000°C.) normally employed for ammonia oxidation.

U.S. Pat. No. 4,963,521 describes an exhaust gas catalyst which, in oneembodiment, is formed of honeycomb cordierite coated with a first layerof gamma alumina mixed with minor proportion of zirconia and ceria, anda second layer formed of cobalt oxide or platinum, rhodium andpalladium.

No mention is made that the catalyst can be used for oxidizing ammonia.

U.S. Pat. No. 5,217,939 describes an ammonia oxidation catalyst obtainedby coating a reticulated foamed ceramic or metal substrate with cobaltoxide or with a noble metal.

The coating is obtained by immersion of the foamed substrate in asolution of a carboxylate of cobalt, or of a noble metal, removing thesubstrate from the solution and calcinating at temperatures from 260° to800° C. (at which the cobalt carboxylate is converted into oxide and thecarboxylate of the noble metal is reduced to the metal).

The ceramic foams have a number of pores per linear inch of 10 to 100(4-40/cm); 30 pores in the examples.

The conversion of ammonia to NO using the foam-Co oxide coated is92-95%; using the foam coated with Pt 97-100%.

Further processing is necessary to convert nitric oxide (NO to NO₂) andthen to nitric acid.

More efficient and economical catalysts in the production of nitric acidare therefore needed.

More efficient catalysts producing relatively high yields of NO₂ aredescribed in U.S. Pat. No. 5,690,900. The catalysts are formed of porousceramic (200-600 cells per square inch i.e. 14-24 cells per linearinch=5-9 cells/cm) coated with at least three layers: the first isformed of alumina with minor proportion of ceria and zirconia, thesecond of oxides of cobalt , zirconium and cerium, the third of platinummetal.

The layers are obtained by immersing the porous ceramic body in asuspension of alumina, zirconium oxide and cerium nitrate, removing theimpregnated ceramic structure and calcinating at 600° to 1000° C.

The resulting surface area is 80-120 m²/g.

The thus coated ceramic is then immersed in a solution of cobaltacetate, cerium nitrate and zirconium acetate, removed from thesolution, and calcinated at 600-1000° C.

The last layer is obtained by immersion in a solution of platinumoxalate, and calcination at 600-1000° C. of the structure removed fromthe solution of the previous treatment.

The ammonia conversion to NO/NO₂is 95-100% with NO/NO₂ ratio of 75/25 to60/40.

U.S. Pat. No. 4,820,678 describes honeycomb-like alloy tapes coated witha catalyst having the perovskite (ABO₃) or spinel (A₂BO₄) structure,wherein A comprises cations of rare earth metal, B, in the perovskitestructure, is selected from manganese, copper and nickel cations, in thespinel structure is selected from iron and nickel.

The catalyst is used for the purification of industrial waste gases,exhaust gases from automobiles, and purification of air.

The honeycomb tapes are obtained by perforating Fe—Cr—Al or Fe—Ni—Alalloy strips of about 0.05-0.12 mm thickness at distance of about1.1-1.2 mm apart to form small holes of 0.4×0.4 mm.

OBJECTS

It is an object of the present invention to provide ammonia oxidationcatalysts comprised in honeycomb-type structures (wherein, due to theirtubular structure, no radial gas flow mixing occurs and therefore nosatisfactory conversion of the reagents) capable of giving high catalystperformance in terms of activity and selectivity and high specificproductivity referred to the volume of the honeycomb structure bed andthe weight of the catalyst.

SUMMARY OF THE INVENTION

The ammonia oxidation catalysts of the present invention are formed ofone or more unit structures each of which formed of a pair of blockshaving honeycomb-type structure comprising catalyst material, the blockshaving interplaced between them a layer of gas permeable materialwherein a radial mixing of the reagent gas flow is obtained.

DETAILED DESCRIPTION OF THE INVENTION

The blocks have height of not more than 15 cm, preferably not more than10 and more preferably not more than 6 cm and more than 2 cm. The heightof the interplaced layer is more than 0.5 cm and not more than 3 cmpreferably not more than 2 cm.

A foamed material having open, randomly connected cells is usable toform the interplaced layer.

For honeycomb-type structure it is meant a structure formed of tubularnot interconnected through bores.

As indicated, in the above structure the interplaced layer performs thefunction of thoroughly mixing the gas flow exiting the first honeycombblock wherein, due to laminar piston-type flow inside the tubular notinterconnected through bores, no radial mixing occurs, thus allowingbetter conversion inside the second block.

The laminar piston-type flow inside the tubular through bores of theblocks favors the maintenance of constant reagents concentration atcontact with the catalyst covering the walls of the through bores.

The material of honeycomb-type block preferably is ceramic or metallic;any other material resistant to the high temperatures of the ammoniaoxidation reaction can also be used. For example, the material of thehoneycomb type block can be a perovskite of the ABO₃ type wherein A is arare earth-element or an alkaline-earth element or mixtures thereof andB is a transition metal element or mixtures thereof.

The density of the cells ranges from 3 to 10 cells/cm; that of the poresof the foamed material is of 4 to 20/cm.

Commercial honeycomb-type blocks are available from Emitech-Germany;commercial foams from Hi-Tech Ceramics—NY, USA.

Foamed alpha alumina and reticulated foams with open cells randomlyconnected are preferred.

Usable honeycomb-type alloy blocks can also be obtained from tapesprepared according to U.S. Pat. No. 4,820,678.

Any type of ammonia oxidation catalyst can be used in the honeycomb-typestructure unit according to the invention. The final catalyst can beobtained by either

-   -   a) coating an inert honeycomb monolithic structure with the        active element or    -   b) extruding the active element powder to a honeycomb monolithic        type structure.

A preferred catalyst comprises mixed oxides of cobalt, manganese andrare earth metals having composition expressed as percentage by weightof Co O, Mn O and rare earth oxide in the lowest state of valence asfollows: 20-45% Co O, 50-60% Mn O, 0.5-20% rare earth metal oxide,preferably La₂O₃ and its mixtures with CeO₂. The mixed oxides aresupported on porous inorganic metal oxides, preferably gamma alumina.Catalysts of this type containing Cu O in place of Co O, and thepreparation thereof are described in WO 2008-090450.

Examples of other usable catalysts are described in U.S. Pat. Nos.5,217,939 and 5,690,900, and WO 99/25650. Other examples are theperovskite type catalyst (ABO₃) and the spinel type (AB₂O₄).

The catalyst unit allows to obtain high conversion of ammonia to NO.

The oxidation reaction conditions are: temperature from 200° C. to 900°C., pressure 1 to 12 bar abs., GHSV 8.000-140.000 h⁻¹.

The unit, thanks to its specific structure offers advantages withrespect to the back pressure and increases the space-time yield sincethe throughput of the existing plants can be more heavily loaded.

Example 1

A catalyst unit formed of three units of honeycomb-like structureceramic blocks each having 5 cm height, 62 cells/cm² and including twolayers of foamed alfa alumina each 2 cm thick interplaced by alternatingone block of monolith and one foam until the five structures arearranged in cascade, was prepared by immersing the blocks in a slurrycontaining gamma alumina milled to 1 to 10 μm having supported on it amixed oxides catalyst of composition expressed in percentage by weightof Co O, Mn O and La₂O₃ of 37.4% Co O, 53.4% Mn O and 9.2% La₂O₃. Thesupported catalyst comprised also Pt in amount of 0.1-0.2 wt %. Saidtransition metal oxides are supported on gamma alumina in a globalamount equal to 20% by weight. The slurry is further composed ofdeionized water and made acidic to pH 4 with acetic acid.

The catalyst was prepared by impregnating gamma alumina with an aqueoussolution of lanthanum nitrate (La(NO₃)₃).

The impregnated support was then dried at 110° C., calcined at 600° C.and thereafter impregnated with an aqueous solution of manganese nitrate(Mn(NO₃)₂), cobalt nitrate (Co(NO₃)₂) and Pt(NH₃)₄Cl₂, and dried at 120°C.

A volume of solution equal to 100% of the pore volume of alumina wasused for impregnation.

The immersed blocks were removed from the slurry and calcinated at 500°C. to obtain reduction of platinum ions to metal.

The thus obtained unit was inserted into a reactor for ammoniaoxidation. The reaction conditions were: GHSV=10.000 h⁻¹, temperature ofthe gas mixtures taken at the inlet of the catalyst unit 550° C.,pressure 1 bar and ammonia concentration equal to 1% v/v in air.

The conversion of ammonia to NO was higher than 96%.

Comparison Example 1

A catalyst unit similar to that used in Example 1, but not comprisingthe foamed alumina layers, was used in a test of ammonia oxidationcarried out under the same conditions as in Example 1.

The conversion of ammonia to NO was 87%.

Comparison Example 2

A catalyst unit formed of a single honeycomb-like structure ceramicblock having 15 cm height, 62 cells/cm2 was prepared according to theprocedure already described in the EXAMPLE 1 and was used in a test ofammonia oxidation carried out under the same conditions as in Example 1.

The conversion of ammonia to NO was 74%.

The disclosures in European Patent Application No. 08172820 from whichthis application claims priority are incorporated herein by reference.

What is claimed is:
 1. A catalyst for ammonia oxidation formed of one ormore catalytic units each of which formed of a pair of blocks having ahoneycomb structure containing tubular passages not interconnectedthrough bores, comprising an ammonia oxidation catalyst, each of saidblocks having height of less than 15 cm and more than 2 cm andinterplaced between them a foamed layer having a height of not more than3 cm and at least of 0.5 cm, formed of foamed material having open,randomly connected cells.
 2. The catalyst according to claim 1, whereinthe interplaced foamed layer has a height of 0.5-2 cm.
 3. The catalystaccording to claim 1, wherein the height of each of the blocks is from 2to 6 cm.
 4. The catalyst according to claim 1, wherein the density ofthe cells of the foamed layer ranges from 3 to 10 cells/cm.
 5. Thecatalyst according to claim 1, wherein the blocks are formed of ceramicor metallic material.
 6. The catalyst according to claim 1, wherein theblocks have a number of cells per cm, which can be the same ordifferent, from 3 to 10 and wherein the number of pores per cm of thefoamed layer is 4 to
 20. 7. The catalyst according to claim 1, whereinthe foamed layer is formed of foamed alumina.